gas sensor

I would think you have over filled the tank and the gas expanded while in the tank and it has pushed past the gasket and through the sending unit. You could try siphoning some of the gas out of the tank then check the tightness on all the screws around the sending unit, I will bet the are loose. I would contact Vintage Speedster and see what they say. Good luck, I am sure this won't be too hard a fix.

i concur...overfilled....the gas cap gasket is junk....i don't overfill and still get a whiff of fumes each time...more so on a curvy bit of road 

I use little PTFE or nylon seals on the screws. They'll definitely leak otherwise.

Open the hood and park it in the sun, it will evaporate.

There is a gasket under the sender and there should be a nylon washer under each screw head. If there aren't get this. Tighten the screws gently; don't deform the nylon washers too much. 

thanks the nylon looks compromised , i will try that thanks all

One may have a copper washer instead of nylon.  That one would be for the ground wire.

This problem doesn't necessarily mean your tank is 'overfilled' .

If I top up my tank to what I consider a 'normal' level (by having the gas pump nozzle click off automatically), the gas sender ends up submerged. The sender gasket and screw washers shouldn't leak in that condition. (I also have a VS, by the way.)

Most of our cars have a vent line attached near the top of the filler tube that leads under the car. That allows air in so gas feeds freely to the engine, but it also allows for fuel expansion and overflow.

The vent line can cause other problems if not properly routed, but that's a topic for another day.